The mammalian preimplantation embryo has been shown to have histamine forming capacity and interference with this function interrupts embryo development and implantation. It has been postulated that histamine produced by the embryo stimulates phospholipase A2 activity thereby stimulating release of free arachidonic acid for increased synthesis of prostaglandins in the embryo or in the endometrium at the site of implantation. Histamine or prostaglandins or both, in a cascade manner, may participate in implantation via cyclic nucleotides.